Process for producing foamed metal

ABSTRACT

FOAMED METALS HAVING IMPROVED CELLULAR STRUCTURE ARE PRODUCED BY MELTING THE METAL, THICKENING THE MOLETEN METAL WITH A GASEOUS VISCOSITY INCREASING AGENT, DEGASSING THE THICKENED METAL, AND FOAMING THE DEGASSED, THICKENED METAL WITH A HEAT DECOMPOSABLE FOAMING AGENT. DEGASSING IS ACCOMPLISHED BY EITHER COOLING THE MOLTEN METAL JUST TO SOLIDIFICATION SO THAT RESIDUAL THICKENING GAS IS SQUEEZED OUT BY METAL CONTRACTION, OR THE MOLTEN METAL IS SUBJECTED TO A VACUUM WHICH REMOVES RESIDUAL THICKENING GAS.

United States Patent O 3,692,513 PROQESS FOR PRODUCING FOAMED METAL Crayton G. Hall, Baton Rouge, La., assignor to Ethyl Corporation, New York, N.Y. No Drawing. Filed Oct. 30, 1970, Ser. No. 85,788 Int. Cl. C22c 1/08, 21/00, 23/00 US. Cl. 75-20 F 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In the production of foamed metal, that is, metal having a plurality of randomly dispersed closed cells throughout a metal matrix, a preferred method is to use a heat decomposable foaming agent to generate the gas to form the cells. This technique is disclosed in US. Pats. Nos. 2,751,289; 2,895,819; 2,983,597; 3,300,296; and 3,297,- 431.

Prior art forms frequently have cells which are of nonuniform structure or undesirable large size. This problem has been to some extent alleviated by increasing the viscosity of the molten metal to aid in the subsequent blowing step. Decreased fluidity, i.e. thickening, of the molten metal enables the foaming operation to be relatively prolonged and the foamed metal to be maintained in its heated, fluid condition without collapsing for relatively prolonged periods since the trapped bubbles cannot readily escape from the thickened melt.

However, the use of a thickening agent has some side effects which might desirably be eliminated. Thus, once thickening has been accomplished there is no further need for residual thickening gas which to an extent defeats the very purpose for which it is employed, namely, its presence adversely affects the uniformity and size of the cellular structure. Therefore the present invention is di rected to solving this problem with residual gas remaining after the thickening step. The following description of the invention will demonstrate how this is accomplished.

SUMMARY OF THE INVENTION The present invention provides a process for foaming a metal or its alloy by at least partially degassing the metal before it is foamed. The degassing is effected after the metal or its alloy has been melted and thickened by introduction thereinto of a gas selected from air, oxygen, or carbon dioxide. Degassing is conducted by solidifying the molten metal or its alloys by a step of cooling just to solidification or by subjecting the molten metal or its alloy to at least a partial vacuum.

A preferred metal is aluminum or its alloys. Preferred aluminum alloys have from about 2 to about percent by weight magnesium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention pertains to the discovery that a foamed metal or metal alloy having more uniform pore structure can be achieved by first thickening the foam with a 3,692,513 Patented Sept. 19, 1972 thickening agent, more particularly described hereinafter, and then degassing the foam subsequent to the thickening step by either cooling the molten metal or its alloy just to solidification or by subjecting the molten metal or its alloy to vacuum.

Among the metals suitable for use in this invention, such as aluminum, magnesium, titanium, and the like, aluminum is preferred. Aluminum alloys are highly preferred and especially desirable are alloys of aluminum with magnesium.

The first step in the practice of the process of this invention is to melt the metal or its alloy. This may be achieved with any suitable apparatus well known in the art. It is desirable that such apparatus be susceptible to being maintained under an inert gas purge. Gases such as nitrogen are especially suitable for this purge. If a melting pot is employed, the pot is raised to a temperature well above the temperature at which the metal or its alloy normally becomes a liquid. This facilitates quickly melting the metal or its alloy inasmuch as the temperature is allowed to slowly recede as the metal or its alloy becomes completely liquid. Desirably the temperature of the metal or its alloy is stabilized to the point of about 10 F. to about 50 F. above its melting point in order to insure that it will remain liquid during the remainder of process.

Once the metal or its alloy has been liquified, it is agitated or stirred by means known in the art, e.g. an impeller, propeller, turbine, or the like. Where a turbine is employed, a rotation rate of r.p.m. to 10,000 r.p.m. is suitable although 3,000 r.p.m. is generally preferred. It also appears useful to employ a slower rate of stirring before the foaming agent, described hereinbelow, is added to the molten metal or its alloy.

Once stirring or agitation of the molten metal or its alloy is well under way, the viscosity increasing agent is added to the molten material. Viscosity increasing agents of this invention include air, oxygen, and carbon dioxide. While the mechanism by which these agents increase the viscosity of molten metals or their alloys is unknown, it is speculated in the case of oxygen that nucleation sites are created in the metal which function to increase its viscosity. Of course, the mechanism may be different in the case of each agent. Particularly, in the case of carbon dioxide, which is theorized to break into carbon monoxide and oxygen, other features of the foamed metal, such as brightness, may be altered.

The physical state of the viscosity increasing agent is not critical inasmuch as these compounds can be employed as solids, liquids, or gases. Thus, the thickening agent may be employed in the physical state which is most convenient which in the case of carbon dioxide may be in solid form and in the case of oxygen in the gaseous form.

On the other hand, it is critical that the viscosity increasing agent be uniformly admixed within the molten metal or its alloy. Thus, it is not enough to treat only the surface of the molten material. The thickening agent must be blended uniformly into the molten material by agitation or stirring at the rates above described.

It is suitable to use any quantity of thickening agent which meets the purposes of this invention, this quantity being of course dependent upon the particular thickening agent selected, the particular metal or metal alloy being foamed, process conditions, and the type of apparatus employed for the process. Generally, it is suitable to use a quantity of thickening agent within the range of from about 0.02 pound or less to about 10 pounds or more of agent per 100 pounds of the metal or its alloy and preferable to use from about 0.1 to about 1 pound of agent per 100 pounds of the metal or its alloy. More specifically, it is suitable to use from about 0.02 pound or less to about 1 pound or more of oxygen per 100 pounds of metal or its alloy, preferable to use from about 0.07 to about 0.5 pound of oxygen per 100 pounds of metal or its alloy, and more preferable to use from about 0.1 to about 0.2 pound of oxygen per 100 pounds of metal or its alloy. It is suitable to use from about 0.2 pound or less to about pounds or more of air per 100 pounds of metal or its alloy, preferable to use from about 0.7 to about 5 pounds of air per 100 pounds of metal or its alloy, and more preferable to use from about 1 to about 2 pounds of air per 100 pounds of metal or its alloy. It is suitable to use from about 0.05 pound or less to about 4 pounds or more of carbon dioxide per 100 pounds of metal or its alloy, preferable to use from about 0.1 to about 1 pound of carbon dioxide per 100 pounds of metal or its alloy, and more preferable to use from about 0.3 to about 0.7 pound of carbon dioxide per 100 pounds of metal or its alloy.

Preferably, the viscosity increasing agent is added to the molten material at a rapid rate. The time period of addition may vary from about 5 seconds to about minutes and is subject to lengthening where very large quantities of thickening agent are employed.

Generally, the pressure at which the viscosity increasing agent is added to the molten metal or its alloy is not highly significant. Subatmospheric, super-atmospheric or ambient pressures can be used although ambient pressure is preferred for reasons of economics. However, high pressure may tend to favorably force a. gaseous agent into the molten metal or its alloy while the associated closed vessel retards escape of the viscosity increasing agent.

After the addition of thickening agent is complete, the molten metal or its alloy is now ready for degassing. Degassing is of course directed to the removal of any residual gas which remains in the molten metal which would, if allowed to remain, adversely affect the uniformity of pore size and substantially reduce predictability of pore size. Such residual gas can be removed in accordance with the present invention by two methods. Thus, the molten metal may be cooled just enough so that it solidifies and upon contracting in the solidification process squeezes out any residual gas. Another method by which residual gas in the molten metal can be removed is by use of a vacuum.

In the technique whereby the molten metal is cooled in order to degas, it is only necessary that cooling be conducted down to the point whereat the molten metal or its alloy solidifies. This temperature of course differs for different metals and their alloys. Further reduction of temperature is not necessary or for that matter even desirable inasmuch as the metal or its alloy must be reheated to the melting point before the blowing agent can be added to the metal or its alloy.

The vacuum technique is to some extent preferable to the cooling technique for degasification. Thus, with a vacuum, the expense of reheating the metal or its alloy after cooling is not required. In addition, the use of a vacuum is quicker than cooling the molten material and reheating it. Any amount of vacuum is desirable and indeed a complete vacuum would be highly preferred. However, the amount of vacuum employed is dependent upon economics and can be adjusted as desired, it being understood that the higher the vacuum the faster and more complete the degassing. One disadvantage of the use of a vacuum is the necessity for more expensive equipment.

After the metal or its alloy has been subjected to degassing, it is brought back up to a molten temperature by the means described above, provided of course a cooling technique has been employed for degassing. Once the metal has been liquified, it is ready for the foaming step. A wide variety of blowing agents can be used in the foaming process of this invention. Broadly, all blowing agents described in the prior art are suitable although some blowing agents are better than other. However, Whatever the blowing agent, the foams of this invention have smaller, more uniform pore size than foams produced from the same metal substrate which have not been made more viscous by the thickening agents above described and then degassed.

Among the various blowing agents, the metal hydrides are preferred, among which titanium, hafnium, or zirconium hydrides, especially the latter, are most preferred. Dihydrides and annealed hydrides of less than stoichiometric composition also can be employed. Generally, the best hydride blowing agents are those which decompose to yield gaseous hydrogen at the temperature of the metal or its alloy which is to be foamed and release hydrogen at a relatively slow rate.

The amount of foaming desired determines the amount of hydride or other blowing agent employed; that is, for a dense foam less blowing agent is used than for a lighter foam. Usually, it is preferred to make foams having a 20 percent density or less, or to make foams which weigh no more than about 20 percent of the weight per given volume of the unexpanded metal. For such foams it is suitable to employ from about 0.5 to about 2 pounds of hafnium hydride, titanium hydride, or zirconium hydride for each pounds of molten metal or its alloy to be foamed. A preferred range is from about 0.6 to about 1.2 pounds per 100 pounds of molten metal to be foamed.

In the foaming step, a temperature is employed which is above that at which the metal or its alloy to be expanded is molten and above the temperature required to thermally decompose the blowing agent. The temperature, however, must not be so high that the blowing gas is released so fast as to cause foaming at an uncontrollable rate. Thus it is preferred to have the temperature of the molten metal or its alloy comparatively cool. Ideally, a temperature is employed at which the melt is just viscous. Taking all these factors into consideration, the metal or its alloy is foamed at temperatures within the range from about l,l30 F. to about 1,250" F. and preferably from about 1,150 F. to about l,200 F., being dependent of course upon the metal or its alloy used.

Generally, it is suitable to carry out the foaming process at ambient pressure although greater or lesser pressures can be employed with better results under certain circumstances. Lower pressures can be deleterious since they can encourage evolution of blowing gas outside the confines of the mass to be foamed. Super-atmospheric pressures up to 1,500 p.s.i.g. or higher can be used.

'In carrying out the blowing step above described, the foaming agent is preferably admixed with the molten metal or its alloy to be foamed by using the agitating or stirring means earlier set forth. In the course of such stirring or agitation, the rate is preferably increased above the initial agitation rate at which time the foaming agent is added. Without exception, the more uniform the mixing, the better the foam. All techniques of mixing known in the art which ensure eflicient mixing of materials and liquids can be employed. Preferably the mixing step is performed in as short a time as is feasible to achieve uniform mixing. For best results with a typical mixture of blowing agent and molten metal or its alloy, sufficient mixing achieves homogeneity within about 10 seconds. This time period may require an agitation rate with a stirring device of up to 10,000 rpm.

It is generally preferable that the addition of foaming agent be at a lower temperature than the addition of the viscosity increasing agent. Accordingly, it is preferred to cool the viscous metal before adding the foaming agent. Frequently, the cooling is best carried out in a second vessel, Le, a vessel other than the hot chamber in which viscosity was increased. The second vessel is generally preheated to within plus or minus 50 (1., preferably plus or minus 20 C. of the foaming temperature, whereupon the viscous material is added thereto.

Foaming temperature is dependent to an extent upon the viscosity of the molten metal or its alloy. For those metals or alloys which are highly viscous, high temperatures are preferentially employed. Where a portion of the alloy, e.g. magnesium, increases the viscosity, foaming temperature will vary directly with the concentration of such portion. On the other hand, some alloys are characterized by having greater fluidities than pure aluminum. Of course, it is feasible to use higher temperatures if fluidity is decreased by the addition of more thickening agent.

Subsequent to the addition of the blowing agent, the molten metal or its alloy is allowed to foam. Foaming may occur within an open or closed mold. The size of closed foaming chambers relative to the quantity of metal or its alloy determines density of the product. Regulation of the mold temperature determines the smoothness and thickness of the skin on the finished article.

Having thus described the invention, the following examples are presented as being illustrative and not limiting of the invention.

Example I A melt pot was charged with pounds of metal alloy which was 7 percent magnesium and approximatel 93 percent aluminum. The pot was heated until the alloy was melted. Oxygen was introduced into the melted alloy with an air motor at a pressure of p.s.i.g., and the melt was simultaneously agitated with a turbine at 3,000 r.p.m. The rate of introduction of oxygen and corresponding temperature of the metal were as follows:

Air motor 0 feed rate Temp pressure Elapsed time (min). (s.c.f.h.) F.) (p.s.l.g.)

When the addition of oxygen was completed, the alloy was cooled to its solidification temperature. Then, the pot was reheated until the alloy was melted and elevated to a temperature of 1160 F. The melt was agitated at 6,000 r.p.m. and 60 grams of powdered zirconium hydride was added. The alloy foamed and filled the pot. After the foam had cooled, it was inspected and found to have cells which were of small size and very uniform structure.

Example II Example I above is repeated except the cooling step is omitted and the melt instead is subjected to a vacuum of below 100 millimeters of Hg while the oxygen is being introduced. Once the addition of oxygen is completed, agitation temporarily is terminated. Results from this example are found to compare favorably with the results of Example 1.

Example III Example I is repeated except air is substituted for oxygen. Results are found to be excellent but not as good as in Example I.

Example IV Example I is again repeated except carbon dioxide is substituted for oxygen. Results are again excellent but not as good as in Example I.

I claim:

1. In a process for foaming a molten metal or molten metal alloy, comprising the steps of, melting the metal or alloy, thickening the molten metal or metal alloy with a gaseous viscosity increasing agent, and foaming the thickened metal or metal alloy with a. heat decomposable foaming agent, the improvement comprising at least partially degassing the molten metal or molten metal alloy before foaming, said degassing being effected after the metal or metal alloy has been liquefied and thickened by introduction thereinto of a gas selected from the group consisting of air, oxygen, and carbon dioxide.

2. The process of claim 1 wherein the degassing comprises cooling the molten metal or molten metal alloy just to solidification.

3. The process of claim 1 wherein the degassing comprises subjecting the molten metal or molten metal alloy to at least a partial vacuum.

4. The process of claim 1 wherein the metal is aluminum.

5. The process of claim 1 wherein the metal alloy is an aluminum alloy.

6. The process of claim 5 wherein the aluminum is alloyed with magnesium.

References Cited UNITED STATES PATENTS 1,919,730 7/1933 Koenig 20 F 3,214,265 lO/1965 Fiedler 752O F 3,305,902 2/1967 Bjorksten 75-2O F X 3,379,517 4/1968 Graper 75-20 F L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner 

